The present invention relates to an image density adjustment method to be employed in electrophotographic copying apparatus.
Conventionally, the following image density adjustment methods are known for use in electrophotographic copying apparatus: (1) the density of copied images is adjusted by changing the development bias potential while, at the exposure to be applied to a photoconductor is kept constant; (2) the image density of copied images is adjusted by changing the exposure while the development bias voltage is kept constant; (3) when the images density of copied image is decreased, the exposure is increased while, at the same time, the development bias potential kept constant and, when the image density is increased, the development bias potential is decreased while, at the same time the exposure is kept constant.
However, when the density of the background of a copied image is increased in order to reproduce a poor original clearly, fatigue of the photoconductor appears on the copied image in the above-mentioned methods (1) and (3) since the development bias potential is lowered while the exposure kept constant. In FIG. 1, an initial light decay curve of the background potential on a photoconductor is indicated by a solid line, and the light decay curve when the photoconductor is subject to fatigue is indicated by a broken line. Assuming that development is performed at the time T.sub.0, the difference between the initial background potential and the background potential at the time of fatigue cannot be developed at a standard development potential V.sub.0. Accordingly, the background does not appear on the copied image. However, when the development bias potential is reduced to V.sub.1 in order to increase the density of the background of copied image, the difference between the initial background potential and the background potential at the time of fatigue is developed and the difference appears in the form of a ghost image on the copied image. Furthermore, when the image density is lowered in order to eliminate the background as in the above-mentioned methods (2) and (3), the fact that the exposure is increased while the development bias potential is kept constant causes fatigue of the photoconductor to be intensified so that the fatigue is reflected in the quality of the copied image.
FIG. 2 shows a light decay curve that results when a standard exposure is applied to the photoconductor and FIG. 3 shows the light decay curve that results when the exposure to be applied to the photoconductor is increased in order to decrease the image density of copied image. When the exposure is increased in order to decrease the image density of copied image, the difference between the light decay curve of the initial background potential and that of the background potential at the time of fatigue of the photoconductor becomes so great that the difference appears on the copied images even if the development bias potential is not changed.